Alkali or alkaline earth metal ferrates, their preparation and their industrial applications

ABSTRACT

The ferrates in solid state have the formula: 
     
         M (Fe, X)O.sub.4 
    
     in which M designates two atoms of Na or K or one atom of Ca or Ba, and X is an atom whose cation has the electronic structure of a rare gas. These ferrates are isomorphs of ferrates of the formula M FeO 4 , in which M has the same definition as above. Used particularly for the treatment of water, by oxidation.

The present invention relates to new alkali or alkaline earth metalferrates.

The invention is also concerned with their process of preparation andtheir industrial applications.

It is known that the ferrates of the formula M FeO₄, in which M is anatom of Ca or Ba, or two atoms of Na or K, are powerful oxidizers.

When they are introduced into water, they liberate oxygen according tothe following reaction:

    2 FeO.sub.4.sup.2- +5 H.sub.2 O⃡2 Fe(OH).sub.3 +4 HO.sup.- +3/2 O.sub.2

However, these ferrates are not stable, so that they cannot be storedfor long periods. This instability is incompatible with industrial useof these ferrates as oxidizers.

The object of the present invention is to provide stable ferrates, whichis to say industrially usable particularly as oxidizers.

According to the invention, these ferrates are characterized in thatthey have the formula:

    M (Fe, X) O.sub.4

in which M designates two atoms of Na or K or one atom of Ca or Ba, andX is an atom whose cation has the electronic structure of a rare gas,said ferrates being isomorphs of ferrates of the formula M FeO₄, inwhich M is as defined above.

Thus the ferrates according to the invention are distinguished fromferrates of the formula M FeO₄ by the fact that certain Fe atoms havebeen replaced by atoms of X.

The formula M(Fe, X)O₄ can be established by classical chemicalanalysis.

It has been established by X-ray diffraction that the ferrates M(Fe,X)O₄ have a crystalline structure identical to that of M FeO₄ ferrates.Only the dimension of the crystal lattice varies slightly because of thepresence of the X atoms, particularly when the dimension of this latteris different from that of the atoms of Fe that they partially replace.

The stability of the ferrates according to the invention can beexplained by the fact that the cation of the atom X has the electronicstructure of a rare gas.

Thus the X atom can be selected from among the following: Al, Si, P, S,Cl, Mo, Mn, Cr and the mixture of these atoms.

The corresponding cations, namely:

Al³⁺, Si⁴⁺, P⁵⁺, S⁶⁺, Cl⁷⁺, Cr⁶⁺, Mo⁶⁺ and Mn⁷⁺ have the electronicstructure of a rare gas.

According to a preferred embodiment of the invention, the ratioFe/(X+Fe) (number of atoms of Fe over that of the (X+Fe) atoms) isbetween 0.5 and 0.9.

When the ferrates according to the invention fulfill this supplementalcondition, they have both a high stability and an excellent oxidizingpower.

According to another aspect of the invention, the process of preparationof the ferrates according to the invention is characterized in thatthere is caused to react the compound M OH and an oxidizer, with achemical compound in which the atoms of Fe are combined with the Xatoms.

The compound M OH can be NaOH, K OH, Ca(OH)₂ or Ba(OH)₂.

Thus, the compound M OH and an oxidizer can be reacted with ironsulphate, iron silicate, iron phosphate, iron chromate, iron molybdate,iron tungstate, iron manganate, iron perchlorate or iron aluminate.

By way of example, to prepare the ferrate of the formula:

    M.sub.2 (Fe, S)O.sub.4

in which M is an atom of Na or K, there is introduced iron sulfate insolid state into a solution of soda or potash, agitation is conductedand the product obtained is dried and an oxidizer is added thereto.

To this end, at least the stoichiometric quantity of iron sulphate iscaused to react with the soda or potash.

According to another embodiment of the process according to theinvention, there is caused to react with the soda or potash a mixture ofiron sulphate and Ca hypochlorite.

The compound of the formula

M₂ (Fe, S)O₄ is the preferred ferrate according to the invention,because it can be obtained from iron sulphate FeSO₄. 7 H₂ O which is aninexpensive chemical product. It is particularly a residue of theindustrial production of titanium.

There will now be developed the theoretical aspects which arefundamental to the present invention.

It is well known that the best known iron salts are those in which themetallic element has degrees of oxidation II and III, namely Fe(II)which has a mildly reducing nature, and Fe(III) which has a mildoxidizing nature.

The degrees of oxidation higher than III are on the other hand much lessknown, whether the corresponding compounds are limited such as BaFeO₃ tothe degree of oxidation IV, or whether the salts are difficult tosynthesize and less stable with time such as K₂ FeO₄ for the degree ofoxidation VI.

It has long been known that K₂ FeO₄ is an isomorph of K₂ CrO₄ or K₂ SO₄,and that likewise BaFeO₄ has the same structure as BaCrO₄ or BaSO₄.

The essential difference between these types of compounds resides intheir stability: thus, although sulphates or chromates are said to bestable in the solid state, nevertheless the ferrates decompose easilyand, because of this, are difficult to handle. These latter also havethe property of oxidizing water with the release of oxygen and theprecipitation of ferric hydroxide; only manganese, in its radical MnO⁻ ₄has the property of oxidizing water with the release of oxygen, but thereaction kinetic is slow such that the crystals of permanganate arestable with time.

Among the ferrates, the radical FeO₄ ²⁻ has a tetrahedral structure.

The concept on which the invention is based consists in stabilizing thistetrahedric radical by replacing a portion of the iron atoms in thesolid state. The stabilization of the tetrahedric radical FeO₄ ²⁻ isobtained by the partial replacement of iron with another X elementexisting in the form XO₄ ^(m-) (m being a whole number), such as SiO₄⁴⁻, PO₄ ³⁻, SO₄ ²⁻, ClO₄ ⁻, CrO₄ ²⁻, MoO₄ ²⁻, WO₄ ²⁻, MnO⁴⁻ and AlO₄ ⁵⁻which are well known for their stability.

The molecular orbital theory applied to a tetrahedric radical XO₄ ^(m-)shows that the electrons of the four oxygen atoms come to occupy all theconnecting orbits σ and π in the radical. The anti-connection energyorbits immediately above are not occupied if the cation X.sup.(8-m)+ hasan electronic structure of a rare gas as for example Al³⁺, Si⁴⁺, P⁵⁺,S⁶⁺, Cl⁷⁺, Cr⁶⁺, Mo⁶⁺ and Mn⁷⁺. On the other hand, the tetrahedron FeO₄²⁻ is destabilized if the cation X.sup.(8-m)+ has d electrons (d¹ forMn⁶⁺, d² for Fe⁶⁺) because in this case the electrons will enter theanti-connection orbits. It will thus be understood that the substitutionof Fe (VI) by a cation with the electronic structure of a rare gasstabilizes the tetrahedral system, because this substitution involves adiminution of the concentration of electrons in the anti-connectionorbits.

There will be given hereafter by way of non-limiting example theoperative manner of preparation of a sulphate according to the inventionhaving the formula:

    K.sub.2 (Fe, S)O.sub.4

There is added to 100 parts by weight of FeSO₄. 7 H₂ O, 70 parts byweight of Ca (ClO)₂ and 200 parts by weight of K OH.

Stirring is conducted for 10 minutes, then oven drying is conducted for3 hours.

There is thus obtained a solid product comprising:

K₂ (Fe, S) O₄, in which the number of atoms of Fe is substantially equalto that of S;

KCl

Ca(OH)₂

The KCl can be separated from the mixture, by washing the latter withpure methanol or containing up to 20% water.

The Ca(OH)₂ is not troublesome for the contemplated use in the presentinvention.

The solid product obtained, freed from KCl, can be stored indefinitelyfor example in drums or bags. This product can therefore be utilized asan oxidizer, particularly for the purification of water, namely drinkingwater and swimming pool water.

When the product according to the invention is placed in contact withwater, the iron precipitates in the form of iron hydroxide and therelease of oxygen oxidizes the organic materials contained in the water,as well as the mineral materials such as the sulphates.

The precipitation of iron hydroxide is beneficial, because this latteracts as a flocculent.

Of course, the invention is not limited to the examples that have beendescribed and there can be given to these numerous modifications withoutdeparting from the scope of the invention.

We claim:
 1. Ferrates in solid state having the formula

    M (Fe, X)O.sub.4

in which M designates two atoms of Na or K or one atom of Ca or Ba, andX is an atom selected from the group consisting of Al, Si, P, S, Cl, Cr,Mo, Mn and a mixture thereof, said ferrates being isomorphs of ferratesof the formula M FeO₄, in which M has the same definition as above,wherein the ratio Fe/(X+Fe) (number of atoms of iron over that of theatoms of (X+Fe) is between 0.5 and 0.9.